1. Field of the Invention
This invention relates to electronic chip testing. More particularly, it relates to non-destructive chip burn-in prior to attachment to a circuitized substrate.
2. Prior Art
Conventionally, chips are tested at the wafer level with "good" chips being subsequently wired into a variety of packages. To weed out early life failures in chips so packaged, the chips are systematically subjected to electrical and environmental stress. Test parameters for such "burn-in" testing are higher than the intended power and temperature expected during normal use but are set at such levels in order to cause failure of weak parts. In practice this technique yields 90%-100% good chips for mature chip programs, and the 0%-10% drop out reduces field failures in actual machines.
Conventional burn-in approaches include several techniques, but in most cases packaged devices are subjected to the process. In one such practice, chips are permanently mounted, burned-in and tested as a final assembly. Parts that fall out during testing must be replaced using burned-in chips removed from other assemblies, which are subsequently scrapped. In another, packaged devices are inserted into connectors on a test board which are placed in a burn-in oven. Good devices are then mounted on a board to form the final assembly.
Attempts have been made to nondestructively test chips. One method uses a fixture with multiple probes to contact chip metallurgy and run tests. IBM Technical Disclosure Bulletin, Vol. 22, No. 4, 9/79, p. 1476 to Bry et al discloses a test fixture requiring no bonding, between chips under test and a substrate. Rather, mechanical forces are used to ensure contact between test probes and electrical terminals of the chip under test. IBM TDB, Vol. 9, No. 8, 1/67, p. 1051 to Chiou et al relates to localized heating of chip bonding pads. Silicon devices are bonded to gold pad on an insulating substrate with high heat. 370 deg. C. and pressure. To selectively remove and replace a silicon device, the pad is locally heated to a temperature in excess of the gold-silicon eutectic temperature. Chips must be subsequently removed by mechanically shearing or by some other technique which frequently damages the solder bumps on the chips. Removal techniques are disclosed in IBM Technical Disclosure Bulletin, Vol 25, No. 9, 2/84, p. 4780 to Formichelli et al. Vol 19 , No. 7, 12/76, p. 2476 to Ward and Vol. 19, No. 7, 12/76, p. 2477 to Angelone.
Each of the above described methods when viewed from the perspective of solder bump integrity, ease of rework and manufacturability has several drawbacks. For instance, once a chip is broken off after burn-in and added to its final assembly, cracks at the junction of the chip pad and C4 bump due to stresses induced when the pad was broken could causes reliability problems when the chip is applied to a final assembly.
Direct chip attach (DCA) techniques eliminate costs associated with first level packaging. In addition, DCA chips may be more densely packaged, yielding a smaller assembly. In order to effectively use DCA assemblies, the devices to be mounted must be burned-in, preferably at the chip level.
Burn-in testing is particularly desirable in these multi-chip module applications. Overall yield in a multi-chip module is a function of individual chip yield (Y) and the number of chips (N). The yield for the multi-chip module is determined by multiplying the chip yield (Y) by itself N times. Detection of chip failures before the assembly process decreases ultimate waste and maximizes overall yield.